A liquid crystal Fresnel lens is proposed that forms a Fresnel lens-like retardation distribution in a liquid crystal layer by utilizing ring-shaped electrodes (refer, for example, to patent document 1).
FIG. 11 is a cross-sectional view of a prior art liquid crystal Fresnel lens.
In the liquid crystal Fresnel lens 30, a liquid crystal 12 is provided between top and bottom transparent substrates 11, and a seal material 13 is placed so as to cover the periphery of the liquid crystal 12. A common electrode 9 and a ring-shaped electrode group 8 are formed on the sides of the top and bottom transparent substrates 11 that face the liquid crystal 12.
In the liquid crystal Fresnel lens 30, the retardation distribution in the liquid crystal 12 is varied by applying voltage between the common electrode 9 and the concentrically segmented ring-shaped electrode group 8 with the voltage being varied for each ring-shaped electrode. The liquid crystal Fresnel lens 30 thus produces a lens effect.
FIG. 12 is a plan view schematically showing the ring-shaped electrode group formed in the prior art liquid crystal Fresnel lens 30.
As shown in FIG. 12, the ring-shaped electrode group 8 comprises a plurality of ring-shaped electrodes 2 arranged concentrically about the center 4. The ring-shaped electrodes 2, which are not necessarily equal in width, are formed in such a manner so as to be separated from each other by a prescribed inter-zone gap 3. In this inter-zone gap 3, no potential is applied to the liquid crystal layer which therefore does not function as a lens in the gap region. It is therefore desirable to form the inter-zone gap 3 as narrow as possible. However, the inter-zone gap 3 is formed with a prescribed width in order to prevent leakage between adjacent ring-shaped electrodes.
Lead wires 5 for applying voltages to the respective ring-shaped electrodes 2 are formed on the ring-shaped electrodes 2 by interposing therebetween an insulating layer not shown. Each lead wire 5 is connected to a designated one of the ring-shaped electrodes 2 by means of a via 6. The lead wires 5 are connected to a driver circuit IC 7. The driver circuit IC 7 is connected to a power supply circuit, etc., not shown, and applies a prescribed voltage to each ring-shaped electrode 2 via a corresponding one of the lead wires 5.
FIG. 13 is a diagram showing the relationship between the ring-shaped electrodes formed in the prior art liquid crystal Fresnel lens 30 and the retardations formed in the liquid crystal layer.
In the liquid crystal Fresnel lens 30 shown in FIGS. 12 and 13, eight ring-shaped electrodes 2 correspond to one lens segment 1. The ring-shaped electrodes 2 belonging to different lens segments 1 but having the same number as counted from the center are supplied with the same voltage via the same lead wire 5 and through the respective via 6.
As shown in FIGS. 12 and 13, the retardation distribution in the lens segment 1(1) is formed by the ring-shaped electrodes 2(1-1) to 2(1-8), the retardation distribution in the lens segment 1(2) is formed by the ring-shaped electrodes 2(2-1) to 2(2-8), and the retardation distribution in the lens segment 1(3) is formed by the ring-shaped electrodes 2(3-1) to 2(3-8). Voltages are applied to the ring-shaped electrodes 2(1-1) to 2(1-8), 2(2-1) to 2(2-8), and 2(3-1) to 2(3-8) via the respective lead wires 5(1) to 5(8). For convenience of illustration, only the lens segments 1(1) and 1(2) are shown in FIG. 12.
The ring-shaped electrodes 2 for forming the retardation distribution in each lens segment 1 are formed in such a manner that the ring-shaped electrodes located farther away from the center have smaller widths. That is, compared with the width of the ring-shaped electrode 2(1-2), the width of the ring-shaped electrode 2(1-3) located farther away from the center is small, and compared with the width of the ring-shaped electrode 2(1-3), the width of the ring-shaped electrode 2(1-4) located farther away from the center is small.
Further, the width of the lens segment 1 across which the retardation distribution is formed becomes smaller as the lens segment 1 is located farther away from the center. That is, the width of the retardation distribution formed by the lens segment 1(1) is wider than the width of the retardation distribution formed by the lens segment 1(2) located farther away from the center.
Patent Document 1: United States Patent Application No. 2005/0231677